Small electric motors are used for many purposes in automobiles. For example, a fully equipped automobile includes individual motors for powering each windshield wiper system, fan, and window lift system and three additional motors for powering each adjustable seat. Electric motors are also used as sunroof, mirror, and antenna drives.
Ordinarily, these motors have metal housings that are formed from drawn steel cans. C-shaped permanent magnets are attached to the sides of the can with adhesives, clips, or screws that are tapped into the magnets. The steel body of the can functions as a flux ring surrounding the magnets. A self-centering bearing is mounted at a closed end of the can. One end of an armature shaft is journaled in the bearing, and the armature shaft can be tilted with the bearing into alignment with another bearing carried in a housing cover.
The drawn steel cans are expensive and add considerable weight to the motors. The self-centering bearings, which are mounted on their outside diameter surfaces, require additional clearance between the armature and the permanent magnets to align the armature shaft between the bearings. The additional clearance detracts from motor performance.
An electric motor having a molded resin housing is disclosed in U.S. Pat. No. 4,412,146 to Futterer et al. A flux return element consisting of two cylindrical half shells is embedded in the periphery of the molded housing and a permanent magnet is embedded about an injection molded element within a central aperture of the housing. A bell-shaped rotor includes a cylindrical winding that rotates between the flux return element and the permanent magnet. Bearing seats for mounting rotor bearings are molded in opposite ends of the injection molded element.
Similar to conventional metal housings, the molded resin housing of Futterer et al. provides for mounting the rotor bearings about their outer diametral surfaces. The bearing seat at the closed end of the housing also forms a separate cavity from the cavity within which the flux return element and magnets are embedded. Accordingly, the design does not allow for the inner diameters of the rotor bearings and flux return element to be accurately aligned to a central axis to optimize motor performance.